1. Field
Embodiments of the present invention relate to the field of microelectronic device fabrication. In particular, an embodiment of the present invention relates to chemical mechanical polishing slurries for aluminum and aluminum alloys.
2. Discussion of Related Art
Chemical mechanical polishing (CMP) techniques are used in the semiconductor industry to remove thin films from the surface of semiconductor substrates. One common use of these techniques is to form metal interconnect lines, vias, or contacts by (a) patterning and etching trenches or holes in an dielectric layer, (b) depositing a blanket of metal, and (c) chemically mechanically polishing away the metal lying over the dielectric layer. FIGS. 1a to 1c illustrate the formation of aluminum interconnect lines using this process.
FIG. 1a shows the cross sectional view of two trenches 110 that have been patterned and etched into a dielectric layer 102. The dielectric layer 102 lies over a silicon substrate 100. FIG. 1b shows the trenches 110 after the blanket deposition of a barrier layer 104 and a layer of aluminum 106. FIG. 1c shows the cross sectional view of two interconnect lines 112 that have been formed after the CMP step is complete.
The apparatus of a typical CMP process is illustrated in FIG. 2. The substrate wafer 200 is placed face down on a polishing pad 212 which is attached to a rotating table 214. In this way, the thin film to be polished is placed in direct contact with the pad 212. A carrier 216 is used to apply a downward pressure F against the backside of the substrate wafer 200. During the polishing process, the carrier as well as the table 214 and pad 212 are rotated while a chemically reactive and abrasive solution, commonly referred to as a “slurry” 222, is pumped onto the surface of the pad 212.
The slurry 222 facilitates the polishing process by chemically reacting with the thin film and by providing an abrasive material to remove the thin film while the table 214 rotates. Slurry composition is an important factor in the manufacturability of the process.
Prior art slurry compositions for the polishing of aluminum contained strong oxidizers such as hydrogen peroxide (H2O2) or persulfate ions (S2O82−) and harsh etchants such as mineral acids or harsh oxidizing ethants such as potassium fluorite (KF). These chemistries were needed to remove thick layers of aluminum oxide created by the strong oxidizers. But, this combination of chemistries leads to chemical attack recessing of the aluminum 103 metal line or via below the surface of the dielectric layer, as shown in FIG. 1c. In the case of KF chemistry, it also leads to chemical attack on the dielectric layer 102. The recessing creates nonplanarities that propagate to subsequent layers and impair the ability to print narrow, high density lines on those layers. The recessing may also affect the integrity of the metal lines or vias formed during the CMP step, presenting reliability problems. For the thin metal thickness used in front end applications, the high recess can lead to complete removal of the Al layer. Additionally, the formation of a thick layer of alumina (Al2O3) by the oxidizing agent produces an increased concentration of alumina. This alumina acts as an aggressive abrasive and causes excessive surface roughness and scratching of the aluminum 103. As the width of the aluminum 103 interconnect lines is reduced to smaller and smaller widths, the affects of recessing, surface roughness, and scratching become increasingly problematic.